1. Field of the Invention
The present invention relates to a method and apparatus for mixing a fluid, particularly a liquid or a gas, whereby the fluid comprises at least one physical and/or chemical fluid property varying along a flow direction of the fluid.
2. Discussion of the Background Art
As shown in previous theoretical and experimental investigations, the fluctuations of the mixing ratio in an eluent flow generated by a high-pressure liquid-cromatography (HPLC-) pump can lead to undesired base line noise. For certain applications, e.g. separations using eluents with UV-absorbing additives, the strict requirements to the constancy or smoothness of the eluent composition over the separation time cannot be satisfied without using additional mixing appliances which disperse the zones with fluctuations of the eluent composition.
Methods and apparatus for mixing liquids or gases, particularly for smoothing fluctuations of physical or chemical properties like temperature and concentration along a flow, are known in the prior art, e.g. in technical areas like liquid chromatography or microfluidics.
In particular, JP 54-128863 A discloses a liquid mixing apparatus for mixing two liquids with different physical properties, e.g a high-temperature fluid with a low-temperature fluid or two fluids differing from each other in density or concentration, in order to obtain mixed fluids comprising an homogenous temperature profile along the axis of flow direction. Hereby a high-temperature liquid is introduced through an inflow port and flows out through an outflow port while throttled by a valve at a short circuit.
In that apparatus, the fluid introduced through the inflow port is split into two partial flows. One of the partial flows is delayed by letting it flow a longer bypass than the other partial flow, and joining both partial flows together again.
The drawback of that known apparatus is that it can mix the two partial flows efficiently only at certain singular frequencies of periodic fluctuations. Further, for its operation a duty cycle of the corresponding fluctuation of approximately 50% is required.
It is therefore an object of the present invention, to provide a method and an apparatus which allow for mixing of a fluid like a liquid or a gas with a predetermined and freely choosable mixing profile along the axis of flow direction.
Another object is to provide a method and an apparatus for smoothing of fluctuations of at least one physical property of such a fluid with a maximum fluctuation-smoothing effect.
A still another object is to provide such a method and apparatus for mixing of the earlier delivered parts of a flow with the following parts of the flow within a certain volume in accordance with any desired mixing function like a Gauss or similar mixing or filtering function.
It is still another object to provide such a method and apparatus which allow mixing with a minimum delay time.
These objects are solved by the features of the independent claims. Advantageous embodiments are subject matter of the subclaims. in particular, the method according to the invention comprises the steps of delaying partial flows of the fluid with different flow delays and providing different flow volumes for the partial flows thus resulting in a predetermined flow distribution function thus determining a dispersion pattern.
The concept underlying the present invention is to let any flow segment join the outlet flow not at once, but distributed over a certain time or volume window, the fraction joining the outlet flow at any moment being determined by the desired mixing function. This allows for a continuous dispersion of the fluid property which is intended to be mixed, for instance to be homogenized, in accordance with a predefined fluid distribution function. Hereby the flow distribution is particularly achieved by moving the fractions of the flow segments with varying flow volumes thus obtaining different segment contributions at different times at the outlet which provide the wished flow distribution function.
In more detail, the proposed continuous mixing is achieved by applying to the fluid the combination of moving partial flows of the fluid with stepwise or continuously different time delays and using different flow volumes for the retarded partial flows. A preferred distribution function is a Gaussian profile. The advantage of a dispersing mixer with a Gauss-function or generally a smooth continuous function is that it is much more efficient than one consisting of only two pathways with an equal cross-section and thus having a mixing function of two narrow peaks, the difference being not only quantitative, but qualitative.
A further advantage over the apparatus according to JP 54-128863 A is that the method of mixing or dispersing fluids according to the invention has not only an optimal efficiency at certain periods of fluctuations but is efficient in a broad range of fluctuation periods as well as for non-periodic fluctuations.
The proposed method allows for a mixing appliance with minimum volume which disperses the zones with fluctuations of the fluid properties according to a desired predefined mixing function. A desired function is achieved by establishing the segment distribution profile and the distance to be covered by different partial flows so that each fluid segment is distributed to a desired profile.
The proposed method further allows to realize a mixing device with immediate response, i.e. the first fractions of a flow segment progress to the outlet immediately.
Computer simulations have shown that, by a given mixing volume, the best fluctuation-smoothing effect can be achieved using a Gauss or similar mixing or filtering function. This can be achieved with the proposed method by dispersing each portion of the eluent to a zone with the desired concentration profile in the outlet flow.
The method can advantageously be realized by splitting the fluid into numerous partial flows with varying flow volumes and by delaying each of the partial flows with a different flow delay.
Hereby the varying of the flow volumes can particularly be realized by a varying hydraulic resistance. Varying of the hydraulic resistance can further be realized by throttling the partial flows, particularly by using flow restrictors. The step of delaying the partial flows with the different flow delay can be provided by different flow lengths, or reservoirs to be flown through.
The proposed apparatus comprises means, arranged between the inlet component and the outlet component for delaying partial flows of the fluid with different flow delays and means for providing different flow volumes for the partial flows wherein the delayed partial flows with different flow volumes correspond to a predetermined flow distribution function.
In one embodiment, the apparatus comprises numerous flow channels, arranged between the inlet component and the outlet component, each with a different channel length and with a varying hydraulic resistance. Hereby the varying hydraulic resistance can be provided by different cross-sections of the respective channels.
According to another embodiment, the apparatus comprises a dispersion chamber, arranged between the inlet component and the outlet component, comprising a cross-sectional shape varying along the flow direction and by a slit fluidly connected to the dispersion chamber, wherein the cross-sectional shape corresponds to a predetermined flow distribution function. The distribution or dispersion, respectively, occurs as a flow progresses through the chamber with a non-uniform cross-section with the flow inlet at one end and with the flow outlet in the form of a narrow slit along one side of the chamber, opening to a flow outlet collector tube.
The chamber can be of uniform cross-section, the distribution function being determined exclusively by the form of the slit, namely the slit width at different distances along the slit.
According to yet another embodiment, the slit can be replaced through a number of restrictor channels, arranged between an inlet channel and an outlet channel with varying distances between each other, wherein the distribution of the restrictor channels with respect to the flow direction in the inlet channel corresponds to the predetermined flow distribution function. Hereby the chamber can be split to a number of smaller chambers of smaller volumes, each being equipped with its own restrictor channel.
All the mentioned method and apparatus features can be implemented in a macro-design for a common physical or chemical application or in a micro-design, e.g. an on-a-chip design for microfluidics applications.